1. Field of the Invention
This invention relates to the field of packet-based networks, such as ATM (asynchronous transfer mode), and more particularly to a method for establishing virtual connections that experience minimal downtime in the network.
2. Description of Related Art
The use of ATM by a continuously widening set of applications is driving a requirement for very robust, highly available connections. Mission critical applications require that ATM virtual connections(VCs) experience very low or no down time when reacting to network failures or network maintenance operations. These ATM virtual connections come in three forms: PVCs (Permanent Virtual Connections), and dynamic on-demand SVCs (Switched Virtual Connections) and SPVCs (Soft Permanent Virtual Connections). SVCs are set up on demand via a signalling protocol and are not automatically re-established after a system restart. SPVCs are signalled PVCs which are set up on demand via a signalling protocol. These PVCs are automatically re-established after a system restart. SVCs and SPVCs enable a network to setup connections on-demand via signaling and routing without the aid of a centrally managed network management platform and are, thus, more robust and efficient than PVCs as the call processing is distributed. SVCs and SPVCs reroute around network failures as soon as the failures are detected. When reacting to network failures, SVCs and SPVCs are tom down, and resignalled and rerouted around the failure. Even though today current ATM connection management strategy using SVCs and SPVCs enjoy an order of magnitude better rerouting performance than a network management platform can provide with PVCs, rerouting performance is still not adequate for mission critical applications where high availability virtual connections are required.
Ryutaro Kawamura et al. in an article entitled xe2x80x9cSelf-Healing ATM Networks Based on Virtual Path Conceptxe2x80x9d in an article in IEEE Journal on Selected Areas in Communications, vol. 12, no. 1, January 1994, pages 120-127 describe a network wherein alternate virtual paths with zero bandwidth are set up between terminating switching nodes for use in the event of a network failure. However, Kawamura only discusses unidirectional paths and teaches that in the event of network failure a restoration message must be sent along the backup VP before transmission can occur. This process can involve significant service interruption.
An object of the invention is to provide an arrangement that that enables VCs to experience low or no down time for VCs recovering from network failures during from network maintenance and optimization operations.
According to the present invention there is provided a method of maintaining communication, in an asynclronous cell-based digital comununications network having a plurality of switching nodes, between first and second endpoints attached to respective endpoint switching nodes during transfer between a primary bi-directional virtual connection established over a first route between said endpoints and a secondary bi-directional virtual connection available over an alternate route through said network comprising the steps of establishing a precursor secondary virtual connection between said first and second endpoints over said alternate route, said precursor secondary virtal connection consisting of unidirectional connections in the receive direction at said endpoint switching nodes and bi-directional connections between said endpoint switching nodes and progressively switching to said secondary bi-directional virtual connection by sequentially establishing a bi-directional connection at said first endpoint switching node, transmitting cells toward the second endpoint switching node over said alternate route, establishing a bi-directional connection at said second endpoint switching node upon receipt of an indication over the alternate mute that the first endpoint switching node has transferred to the alternate route, and transmitting cells toward the first endpoint from the second endpoint over the alternate route to complete the transfer of communication between said first and second endpoints to said secondary bi-directional virtal connection.
This arrangement for the handling of network failures and maintenance operations ensures that a backup or alternate data path exists for each mission critical virtual connection. However, when the alternate data path is setup, the determination of how and when to switch to the alternate path is different depending on whether a network fault is being processed or a maintenance operation is processed. If a maintenance operation is being processed then a first mechanism, known as a Bridge and Roll, is used. If a network reroute is being processed. then a second mechanism, known as a RAPID mechanism, is used.
The establishment of an alternate backup VC combined with an efficient determination and mechanism to switch to the alternate path ensures very high availability of mission critical VCs, much more so than waiting for each VC to be resignalled and routed.